Presenter:Prof. Norbert Koch

                  Institut für Physik & IRIS Adlershof,Humboldt-Universität zu Berlin, Germany 

                  Helmholtz Zentrum Berlin für Materialien undEnergie GmbH, Germany

                 JiangsuKey Laboratory for Carbon-Based Functional Materials & Devices, 

                 Institute of Functional Nano& Soft Materials (FUNSOM), Soochow University, P.R. China

Topic:Acomprehensive and unified picture of energy level alignment at interfaces withorganic

           semiconductors

Time:02:00 PM, Apr. 5^th （Tuesday）

Location:Conference Room B, BLDG 909-1F

Abstract

Controllingthe energy level alignment at the ubiquitous interfaces in modern organic lightemitting diodes, i.e., organic/electrode and organic/organic, is mandatory forachieving highest performance. While for some interfaces the understanding hasmatured over the past years � often with the help of photoelectron spectroscopyinvestigations, a lack of material-overarching and general models seems topersist. In this context, it is interesting to note that photoelectronexperiments reported by different groups often returned a different levelalignment for a given interface, which certainly should be unsettling fordevice engineers. It turns out that Fermi-level pinning and its consequencesfor charge density re-distribution across a device stack is an overarching mechanismthat should always be considered.

First,a generalized picture of how the levels between generic electrodes and organicsemiconductors align is presented, contrasting metal and polymer/oxideelectrode materials. Depending on the electronic coupling strength of theinterfaced materials, an interface dipole and flat bands or gradual energylevel ("band") bending results.

Forintrinsic organic heterojunctions of materials with moderate acceptor/donorcharacter the electrostatic potential across the interface changes onlymarginally � if at all. This situation, however, can be significantly alteredwhen at least one of the two semiconductors is Fermi-level pinned by the"effective work function" of the other one, which is established bythe contact to the electrode. Consequently, device engineering has to fullytake into account the effect of adding the electrodes to a device stack,otherwise correlations between assumed electronic structure and deviceperformance remain uncertain.

Biography